Paper products are used for a variety of purposes. Paper towels, facial tissues, toilet tissues, and the like are in constant use in modern industrialized societies. The large demand for such paper products has created a demand for improved versions of the products. If the paper products such as paper towels, facial tissues, toilet tissues, and the like are to perform their intended tasks and to find wide acceptance, they must possess certain physical characteristics. Among the more important of these characteristics are absorbency, softness, and strength.
Absorbency is the characteristic of the paper that allows the paper to take up and retain fluids, particularly water and aqueous solutions and suspensions. Important not only is the absolute quantity of fluid a given amount of paper will hold, but also the rate at which the paper will absorb the fluid. Softness is the pleasing tactile sensation consumers perceive when they use the paper for its intended purposes. Strength is the ability of a paper web to retain its physical integrity during use.
There is a well-established relationship between strength and density of the web. Therefore efforts have been made to produce highly-densified paper webs. One of such methods is disclosed in the U.S. Pat. No. 4,112,586 issued Sep. 12, 1978; the U.S. Pat. Nos. 4,506,456 and 4,506,457 both issued Mar. 26, 1985; U.S. Pat. No. 4,899,461 issued Feb. 13, 1990; U.S. Pat. No. 4,932,139 issued Jun. 12, 1990; U.S. Pat. No. 5,594,997 issued Jan. 21, 1997, all foregoing patents issued to Lehtinen; and U.S. Pat. No. 4,622,758 issued Nov. 18, 1986 to Lehtinen et al.; U.S. Pat. No. 4,958,444 issued Sep. 25, 1990 to Rautakorpi et al. All the foregoing patents are assigned to Valmet Corporation of Finland and incorporated by reference herein.
Basically, the technology described in the foregoing patents uses, in a representative embodiment, a pair of moving endless bands to dry the web which is pressed and is carried between and in parallel with the bands. The bands have different temperatures. A thermal gradient drives water from the relatively hot band contacting the web towards the relatively cold band contacting the fabric into which the water condenses. While it allows production of a highly-densified, rigid, and strong paper, this method is not adequate to produce a strong and--at the same time--soft paper suitable for such consumer-disposable products as facial tissue, paper towel, napkins, toilet tissue, and the like.
It is well known in the papermaking art that the increase in the density of a paper generally decreases the paper's absorbency and softness characteristics, which are very important for the consumer-disposable product mentioned above. Foreshortening of the paper may provide increases in the paper's caliper, absorbency, and softness. As used herein, foreshortening refers to reduction in length of a dry paper web, resulting from application of energy to the web. Typically, during foreshortening of the web, rearrangement of the fibers in the web occurs, accompanied by at least partial disruption of fiber-to-fiber bonds. Foreshortening can be accomplished in any one of several ways. The most common method is creping, in which method the dried web is adhered to a smooth surface, typically the surface of the Yankee dryer drum, and then removed from the surface with a doctor blade. Such creping is disclosed in commonly-assigned U.S. Pat. No. 4,919,756, issued Apr. 24, 1992 to Sawdai, the disclosure of which patent is incorporated by reference herein. Alternatively or additionally, foreshortening may be accomplished via wet-microcontraction, as taught in commonly-assigned U.S. Pat. No. 4,440,597, issued Apr. 3, 1984 to Wells et al., the disclosure of which patent is incorporated by reference herein.
In any process where the primary purpose is to form a uniformly-densified strong paper (such for example, as a paper board), the use of foreshortening is highly-objectionable. In contrast with the methods for producing uniformly-densified papers, cellulosic structures currently made by the present assignee contain multiple micro-regions defined most typically by differences in density. The differential-density cellulosic structures are created by--first, an application of vacuum pressure to the wet web associated with a papermaking fabric, thereby deflecting a portion of the papermaking fibers to generate low-density micro-regions, and--second, pressing, for a relatively short period of time, portions of the web comprising non-deflected papermaking fibers against a hard surface, such as a surface of a Yankee dryer drum, to form high-density micro-regions. The high-density micro-regions of the resulting cellulosic structure generate strength, while the low-density micro-regions contribute softness, bulk and absorbency.
Such differential density cellulosic structures may be produced using through-air drying papermaking belts comprising a reinforcing structure and a resinous framework, which belts are described in commonly assigned U.S. Pat. No. 4,514,345 issued to Johnson et al. on Apr. 30, 1985; U.S. Pat. No. 4,528,239 issued to Trokhan on Jul. 9, 1985; U.S. Pat. No. 4,529,480 issued to Trokhan on Jul. 16, 1985; U.S. Pat. No. 4,637,859 issued to Trokhan on Jan. 20, 1987; U.S. Pat. No. 5,334,289 issued to Trokhan et al on Aug. 2, 1994. The foregoing patents are incorporated herein by reference.
Now it has been found that soft and, at the same time, strong differential-density paper webs may be successfully produced by first--highly densifying at least selected portions of the web between two press surfaces, and then--creping the web off one of the press surfaces to which the web has adhered during pressing. The commonly assigned co-pending patent applications entitled "Differential Density Cellulosic Structure and Process for Making Same" filed on Jun. 6, 1997 in the name of Trokhan et al., and "Fibrous Structure and Process for Making Same" filed on Aug. 15, 1997 in the name of Trokhan et al. are all incorporated by reference herein. It has also been found that foreshortening of the paper web may beneficially be accomplished in these processes, completely eliminating a need for the Yankee dryer drum as a creping surface.
Accordingly, it is an object of the present invention to provide an apparatus and an improved papermaking process for making a foreshortened strong and--at the same time--soft paper web, eliminating the need for a Yankee dryer.